1. Field of the Invention
The invention pertains to the field of ratcheted tensioning devices for chain driven power transmission systems on internal combustion engines. More particularly, the invention pertains to a ratchet mechanism designed to compensate for the effects of long term wear on the components of the chain drive system.
2. Description of Related Art
A tensioning device, such as a hydraulic tensioner, is used as a control device for a closed loop power transmission chain as the chain travels between a plurality of sprockets that are connected to various operating shafts of an internal combustion engine. In this system, the chain transmits power from a driving shaft to at least one driven shaft, so that at any point in time, part of the chain is slack and part of the chain is tight. The driven shaft(s) could be either one or more camshaft or a balance shaft. It is important to impart and maintain a certain degree of tension on the chain to prevent noise, slippage, or the unmeshing of teeth as in the case of a toothed chain. Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because the jumping of teeth will throw off cam timing, possibly causing serious damage to the engine or rendering it totally inoperative.
However, in the harsh environment of an internal combustion engine, numerous factors cause fluctuations in the tension of any given portion of the chain. For instance, extreme temperature fluctuations and differences in thermal expansion coefficient rates between the various parts of the engine can cause the chain tension to vary from excessively high or very low levels. Over time, the components of the chain drive system will wear, which results in a steady decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tension that are amplified as the various components wear over time. Further, as wear increases, the reverse rotation of the engine, which occurs during stopping or in failed attempts at starting the engine, will result in increasingly severe fluctuations in chain tension. For these reasons, a mechanism is desired to manage the progressive wear of the components of the drive system to insure that the chain and other components of the power transmission system are not subjected to either excessive slack or the over tensioning of the chain.
Hydraulic tensioners are a common method of controlling chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of a chain drive system. The lever arm pushes toward the chain, tightening the chain when the chain is slack. However, it also must provide a resistive force as the chain tightens.
To address these concerns, hydraulic tensioners are commonly used. They typically contain a rod or cylinder as a piston, which is biased in the direction of the chain by a tensioner spring. The piston is contained within a cylindrical housing, having an interior space which is open at the end facing the chain and closed at the other end. The interior space of the housing defines a pressure chamber which is connected to a reservoir or exterior source of pressurized hydraulic fluid via channels or ducts. The pressure chamber is typically formed between the housing and the piston, and it expands or contracts as the piston traverses axially within the housing.
Many tensioners also employ a pressure relief mechanism that allows fluid to exit the pressure chamber when the pressure in the chamber is too high, thus allowing the piston to retract at a controlled rate in response to increasing chain tension. In some tensioners, the pressure relief mechanism is a spring biased check valve. The check valve opens when the pressure exceeds a certain pressure point. Some tensioners may employ a valve that performs both the inlet check function as well as the pressure relief function.
Spring blade tensioners are also known to control chain or belt tension where load fluctuations are not too severe so as to overly stress the spring or springs. A ratchet with a backlash mechanism may be added to such tensioners to limit the backward travel of the tensioning device.
A conventional blade tensioner includes a blade shoe having a curved chain sliding face and at least one blade spring. The spring material is preferably a metallic alloy that is treated to impart spring-like tension to the blade spring. Multiple blade springs may be arranged in layers under the chain sliding face to forcefully bias the blade shoe against the chain. The ends of each blade spring are secured within the blade shoe to resist the unlimited retraction of the blade shoe in response to increases in chain tension. A bracket is provided for mounting the blade tensioner to the engine. The bracket may be allowed to pivot with respect to the chain in response to changes in chain tension.
FIG. 1 shows a tensioner that uses a ratchet device in a chain driven power transmission system. The power transmission system includes a drive shaft 302 having a sprocket 303 that uses a continuous loop chain 306 to drive the sprocket 305 of a driven shaft, such as a camshaft, 304. The ratchet tensioner 301 contains a tensioner housing 307 having a hole 312 for receiving a plunger 308 and a ratchet pawl 317 pivotally mounted about pin 316 to the tensioner housing 307 and biased by a ratchet spring 318. The plunger 308 has teeth on one side of its outer perimeter to engage the ratchet pawl 317. The plunger 308 is biased outward from the hole 312 toward the tension lever 310 by the introduction of pressurized fluid into the hollow chamber 313 and by the force of the plunger spring 314. The tensioner lever 310 pivots on support shaft 309 and has a sliding face 311 that contacts and applies tension to the slack side of the timing chain 306. The rearward movement of the plunger 308 back into the hole 312 is limited by the one way engagement of the ratchet pawl 317 with the teeth on the plunger.
One type of tensioner assembly that might be used to compensate for component wear is shown in FIG. 2. A drive sprocket 102 transmits energy from the engine's crank shaft (not shown) via a belt or chain 100 to at least one driven sprocket 104 (104′). A chain drive is most common. The driven sprockets 104 and 104′ each drive a camshaft of the internal combustion engine, both of which in turn control engine timing. The strands of chain that run from either the drive sprocket to the at least one driven sprocket or vice versa vary in tension in response to different engine conditions. During engine operation, one strand is substantially tighter than the other strand, which is considered slack by comparison. In systems where the chain engages toothed sprockets, a tensioning device 100 is used to prevent skipping of the chain from the sprocket teeth in response to chain tension fluctuations.
The tensioning device 100 is made up of two tensioner brackets 112 and 112′. Each tensioner bracket is pivotally mounted to the engine at a single pivot point 106, 106′, 106a, or 106a′, the specific location being dictated by different design parameters for a specific engine. For example, tensioner bracket 112 may be secured at a first end 107 at pivot point 106 or approximately at a mid point 106a along its length, while tensioner bracket 112′ might be secured to the engine at its first end 107′ at pivot point 106′ or approximately at a mid point 106a′ along its length. The second ends of the tensioner brackets 112 and 112′, designated as 109 and 109′, respectively, are adjustably connected to each other by arm 130. Arm 130 contains a ratchet mechanism 155, which, in this instance is a toothed device designed to tighten in response to excessive slack in the chain or the wearing of the components of the power transmission system over time. The teeth of the ratchet 155 index in only one direction, thus providing a no return function. As the components of the chain drive system wear with time, sudden and severe spikes or surges in chain tension may result in the undesired indexing of at least one tooth. Since the ratchet cannot return to its prior position, this could result in the undesired over-tensioning of the chain when normal operating conditions return.
It is an object of the present invention to provide a ratchet mechanism for a chain tensioner that functions with limited travel under low tension conditions. In this way, the tensioner compensates for the wearing of the various components of the chain drive system over time while tensioning the chain in response to the wide range of fluctuations in chain tension under normal operating conditions. The tensioner must be able to function effectively in the various chain drive systems present within an internal combustion engine, such as, for example, a driveshaft/camshaft system or a balance shaft system.
It is another object of the present invention to provide a less expensive and quieter replacement for conventional ratcheted hydraulic tensioners. The present invention satisfies this objective by providing a device that is more compact than a traditional ratcheted hydraulic tensioner, thus taking up less room in the highly space compromised chain drive compartment of most internal combustion engines.